1. Field of the Invention
The present invention relates to a device for estimating a friction coefficient on road surface of a vehicle in order to accurately estimate the friction coefficient on road surface of a vehicle running on a road.
2. Description of the Related Art
In recent years, a variety of control techniques of a vehicle such as a traction control, a braking force control, and a torque distribution control have been proposed and put in practice. In many of these techniques, a friction coefficient on road surface is used for computing or correcting necessary control parameters and is therefore needed to be accurately estimated in order to reliably perform these controls.
Japanese Unexamined Patent Application Publication No.7-186989 discloses such a technique for estimating a friction coefficient on road surface, in that a deviation of a lateral acceleration which is a difference between an estimated lateral acceleration on the basis of an actual yaw rate of a yaw rate sensor and an actual lateral acceleration of a lateral acceleration sensor is computed, and it is estimated that a friction coefficient on road surface becomes smaller as the deviation of a lateral acceleration becomes greater.
Also, Japanese Unexamined Patent Application Publication No. 8-2274 discloses a technique in that an adaptive control theory is introduced so as to estimate a friction coefficient on road surface on the basis of an equation of motion of a lateral motion of a vehicle by using an angle of steering wheel, a vehicle speed, a yaw rate, and so forth.
Unfortunately, with the technique disclosed in Japanese Unexamined Patent Application Publication No. 7-186989, since estimation accuracy of a friction coefficient on road surface is susceptive to a resolution and noises of each sensor, the sensor has to meet many requirements, for example, its zero-point is effectively corrected, and it is shielded and is fixed at a proper position, in order to maintain a highly accurate output state.
Also, with the technique disclosed in Japanese Unexamined Patent Application Publication No. 8-2274, since a variation of the dynamic characteristic of a vehicle motion due to a difference in friction coefficients on road surface is in principal used, a friction coefficient on road surface is difficult to be detected without a vibratory steering input.
In view of the above problems, the present invention has been made. Accordingly, it is an object of the present invention to provide a device for estimating a friction coefficient on road surface of a vehicle, which minimizes influences of a resolution of a sensor, zero-point correction, noises, and the like, and which accurately estimates the friction coefficient on road surface without a vibratory steering input.